Although the illustrative embodiments will be described in connection with the measurement of signal quality of a downlink transmission in a wideband code division multiple access (WCDMA) cellular telecommunications network the methods described herein should not be considered limited to this type of transmission or network, but should be considered applicable to other devices and network types.
Overestimating Signal to Interference Ratio (SIR) and Signal to Noise Ratio (SNR) measurements can be a significant problem in some communications systems under certain conditions. Overestimation of these parameters can occur when the measured interference/noise power value underestimates the actual interference or noise power value. Improving the accuracy and reducing the variability of signal error and noise power values should reduce the inaccuracies in SIR and SNR measurements and consequently decrease variation in downlink power.
For example, in a WCDMA network, the dedicated physical channel (DPCH) interference signal code power (ISCP) measurements made by a user equipment typically exhibit a very high variation, which can lead to poor inner loop power control performance. The largest source of variation in the DPCH ISCP is usually sampling error, arising from the small number of pilot symbols available for each calculation. Additionally, since the DPCH pilot symbols take up a relatively small proportion of the slot, ISCP measurements based on the DPCH pilot symbols won't pick up interference that is distributed unevenly over the slot. Therefore, it is possible for the ISCP measurements to not reflect the ISCP of the data symbols, and for them to be biased.
In some conditions, biased ISCP measurements will cause the relationship between the measured SIR and block error rate (BLER) to change rapidly, or be significantly different to what it is in normal conditions. This can prevent outer loop power control from being able to control the BLER closely and achieve the desired BLER.